This invention relates to loaded pallets. More specifically, this invention relates to an improved system for stacking loaded pallets one on top of the other.
Pallets, e.g., wood pallets, are used to store and transport loads of a vast range of goods. For example, a pallet can be used to transport boxes of goods that have been stacked and shrink-wrapped or otherwise secured upon the pallet from a manufacturer to a point of sale. Of course, a pallet carrying a full load can be very large and heavy and, therefore, difficult, dangerous and time consuming to move within a warehouse, for example. And oft times two loaded pallets are stacked one on top of the other for handling time and/or storage efficiency reasons.
One type of known pallet moving and stacking system simply employs a first forklift truck to move a first loaded pallet to a stacking location, whereat either the first forklift truck must retrieve a second loaded pallet to stack upon the first loaded pallet, or a second forklift truck is employed to stack the second loaded pallet upon the first loaded pallet. Then, a forklift truck must lift the first and second stacked and loaded pallets from the stacking area and move them to a storage location. This system for moving and stacking pallets is time consuming and can be dangerous. Using a first forklift truck with or without a second forklift truck helping stack loaded pallets requires the time and attention of at least one employee, and is relatively slow. Because using forklifts requires at least one forklift operator, moving and stacking pallets by this system also is prone to human error. For example, an operator could have an accident, toppling an upper loaded pallet from a lower loaded pallet on the forklift, and possibly endangering other employees.
Automated pallet moving and stacking systems are also known in the prior art. In a first type of stacking system, a vertically adjustable fork travels generally horizontally back and forth perpendicular to a conveyor. The fork picks up a first loaded pallet provided on the conveyor, and elevates that pallet above the conveyor. A second loaded pallet is conveyed into position under the elevated first pallet which is then set down upon the second loaded pallet. This system has a large operating footprint. That is, the conveyor and traveling fork system takes up a significant space within a warehouse, along a loading dock, or other location having a large enough floor space which could otherwise be used more efficiently.
In a second known automated pallet stacking system, a conveyor transports a first loaded pallet to a scissors-lift table. The scissors-lift table lifts the first loaded pallet and, in the lifted position, a fork engages the pallet. The scissors-lift table then retracts back to the level of the conveyor. A second loaded pallet is then conveyed to the scissors-lift table. The scissors-lift table raises the second loaded pallet into engagement with the first loaded pallet held by the retractable fork. The fork then retracts from engagement with the first loaded pallet, the scissors-lift table is lowered, and the first and second loaded pallets are thereafter moved away from the scissors-lift table to a desired location. In order to use a scissors-lift table system, an existing conveyor system must be retrofitted to accommodate the scissors-lift table itself. And this system also uses significant floor space because of the retractable fork.
In the above mentioned automated systems which are hydraulic powered, maintenance is time consuming, difficult and messy when dealing with hydraulic fluids. Also, the hydraulic systems are expensive to maintain and require compliance with environmental regulations regarding hydraulic fluid disposal.
It is therefore an objective of the present invention to provide an improved stacking system for loaded pallets whereby the efficiency of stacking loaded pallets is enhanced over prior art systems.
It is a further objective of the present invention to provide an improved system for stacking loaded pallets which occupies less space, i.e., has a smaller floor footprint, than prior art pallet stacking systems.
It is another objective of the present invention to provide an improved stacking system for loaded pallets in which cost and frequency of maintenance is reduced relative to prior art systems.
It is another objective of the present invention to provide an improved system for stacking loaded pallets in which the stacked pallets"" stability characteristics are sensed prior to being stacked one on top of the other.
The objectives of the present invention are realized with a modular pallet stacker for stacking loaded pallets one on top another in which a first loaded pallet is stacked upon a second loaded pallet within a stationary frame defining a stationary work station with a vertically adjustable pallet carriage in operational engagement with the work station. The pallet carriage engages the first loaded pallet at an entry position in the work station, and vertically adjusts the first loaded pallet to a temporary holding position within the work station, which holding position is at a vertical location different from the entry position of the first loaded pallet. The work station then receives a second loaded pallet at the entry position in vertical registration with the first loaded pallet held in the temporary holding position. The pallet carriage is then vertically adjusted until the first loaded pallet is received in stacked relation with the second loaded pallet.
The pallet stacker advantageously uses a series of height and load sensors to ensure that the first and second loaded pallets have the desired stability characteristics prior to being stacked one on top of the other. In particular, a first height sensor checks the height of each of the first and second loaded pallets as each is advanced by a conveyor into the work station. First and second load sensors check that each of the first and second loaded pallets has a preferred load characteristic to ensure that a full load has been previously automatically loaded on each pallet. A second height sensor senses when the first loaded pallet is successfully stacked on the second loaded pallet. This allows the conveyor upon which the first and second stacked pallets rest to transport the stacked pallets away from the work station.
The system also preferably includes an alignment stop engaged by the leading edge of the first and second loaded pallets as each is successively transported by the conveyor into the work station. Each loaded pallet is properly registered or aligned within the work station against the stop because it is pushed against the stop by the conveyor. A stop sensor connected with the conveyor acts to stop the sensor after a time delay sufficient to allow alignment of the loaded pallet in the work station. A carriage grip completion switch and a carriage disengagement switch are also preferably provided on the pallet carriage in order to, respectively, arrest engagement of the pallet carriage with the first pallet and arrest movement of the pallet carriage when the first loaded pallet is stacked upon the second loaded pallet.
A method in accord with this invention for automatically stacking first and second loaded pallets, one on top of the other, is provided by locating a first loaded pallet in an entry position at a stationary work station where a vertically adjustable pallet carriage engages the first loaded pallet and vertically adjusts the first loaded pallet to a height exceeding the first loaded pallet""s height to a temporary holding position above the entry position in the stationary work station. Thereafter, a second loaded pallet is located in the entry position of the stationary work station and the first loaded pallet is moved in a reverse direction downwardly from its temporary holding position into stacked relation with the second loaded pallet. By moving the first loaded pallet in a vertical direction above the entry position of the stationary work station to the temporary holding position and, thereafter, positioning the second loaded pallet in the entry position of the stationary work station to receive the first loaded pallet thereon, the present invention provides a method for stacking loaded pallets which occupies a relatively small floor space footprint when compared to prior art pallet stacking methods. The present inventive method may be employed with an existing conveyor system rather than prior art pallet stacking methods which may require substantial modification to existing conveyor systems.